The basic principle of orthogonal frequency division multiplexing (OFDM), which is a multi-carrier modulation scheme, in a wireless communication system will be described as follows.
In the OFDM scheme, a data stream having a high rate is divided into a large number of data streams having a slow rate and the data streams are simultaneously transmitted using a plurality of carriers. Each of the plurality of carriers is called a sub-carrier.
Since orthogonality exists among the subcarriers, the subcarriers can be detected by a receiving end even in the case that frequency components of the subcarriers overlap with each other. The data stream having the high rate is converted into a plurality of data streams having the slow rate by a serial-to-parallel converter, each of the plurality of data streams converted in parallel is multiplied by each of the subcarriers, the data streams are added to one another, and the added data streams are transmitted to the receiving side.
The plurality of parallel data streams generated by the serial-to-parallel converter can be transmitted by the plurality of sub-carriers using an Inverse Discrete Fourier Transform (IDFT). In this case, the IDFT can be efficiently implemented using an Inverse Fast Fourier Transform (IFFT). Since a symbol duration of each of the subcarriers having the slow rate is increased, a relative signal dispersion on a time axis, which is generated by multi-path delay spread, is decreased.
In the wireless communication using such an OFDM scheme, inter-symbol interference can be reduced in such a manner that a guard interval longer than delay spread of a channel is inserted between OFDM symbols.
In other words, while each symbol is being transmitted through a multi path channel, a guard interval longer than maximum delay spread of a channel is inserted between continuous symbols. At this time, to prevent inter-subcarrier orthogonality from being violated, a signal of the last interval (i.e., guard interval) of an effective symbol interval is copied and attached at a start part of a symbol. This will be referred to as cyclic prefix (CP). The cyclic prefix will be described with reference to FIG. 1.
FIG. 1 is a diagram illustrating an example of a symbol structure which includes a cyclic prefix.
Referring to FIG. 1, a symbol period (Ts) becomes a sum of an effective symbol interval (Tb) and a guard interval (Tg). A receiving end performs demodulation by selecting data corresponding to the effective symbol interval after removing the guard interval. A transmitting side end the receiving end can be synchronized with each other using a cyclic prefix, and can maintain orthogonality between data symbols.
In the wireless communication system to which the OFDM scheme is applied, CPs of different lengths can be used depending on environment to which each cell belongs or properties of data transmitted within the cell. For example, a long CP is used in a cell operated under wireless channel environment where delay spread is great while a short CP is used in a cell operated under wireless channel environment where delay spread is small. For another example, within one cell, data which requires high receiving performance is transmitted using a long CP while data which requires relatively low receiving performance is transmitted using a short CP.
In this way, if CPs of different lengths are transmitted through one transmission unit, for example, one frame, more efficient communication can be performed.